Chemical composition and process for aluminum etching



l 3 entr United States Patent CHEMICAL COMPOSITION AND PROCESS FOR ALUMINUM ETCHING Robert Houston Elliott, Jr., Oreland, Pa., assignor to Pennsalt Chemicals Corporation, a corporation of Pennsylvania N Drawing. Application November 2, 1955 Serial No. 544,582 g Claims. (CI. 41-42) This invention relates to the etching of aluminum alloys and more particularly to the rapid etching of aluminum alloys wherein magnesium is the principal alloying metal, and to compositions for obtaining such rapid etching.

The term etching as used herein, both in the specification and claims, includes not only light surface etching, but chemical dissolution of substantial amounts of the metal treated as occurs when forming metal objects through controlled selective chemical dissolution, a process frequently referred to as chem milling.

The term magnesium-rich aluminum as used herein in the specification and claims is intended to designate aluminunrbase alloys which contain magnesium as their principal alloying element. Thus the invention is applicable to any of the alloys commonly referred to as the fifty series, whose various compositions may be determined by reference to, for example, the Reynolds Metals Companys Aluminum Data Book. The most commonly used alloy of this type is that known as 528, which contains 2.5% magnesium and 97.3% aluminum, with smaller amounts of other metals.

It is frequently desirable in the fabrication of construction parts from aluminum, for example in the fabrication of airplane parts, to produce parts which are relatively light in weight and which still retain relatively high mechanical strength. In orderto reduce the weight of such parts without impairing their strength, it is the practice to remove as much of the metal as possible from the portions of the part which do not contribute to its mechanical strength. It is sometimes extremely diflicult, if not impossible, to accomplish this by the conventional methods of mechanical milling or machining for the reasons that the parts may be highly complex in shape, or that by mechanical removal of the excess metal the strength of the part may be adversely affected. For these reasons, the practice has recently developed of removing this excess metal by chemical means.

It is known that construction parts made from aluminum base alloys can be subjected to an alkaline etching bath, under controlled conditions of concentration and temperature, whereby metal, not protected, is dissolved. The portions of the part from which it is not desired to remove metal are protected by some suitable masking material, which usually consists of a coating of chlorinated neoprene, polyvinyl chloride or latexbase film, or other alkaliresistant coating. In addition, delicately fretted or perforated articles are frequently manufactured by mechanically working two metals, only one of which is soluble in a particular etching bath, into the desired shape and then removing the soluble metal in the etching bath. By these methods highly complex shapes are more easily formed. When large amounts of metal need to be dissolved, it is advantageous to use an etching bath that provides the fastest obtainable rate of attack on the metal.

It has now been discovered that the etching rate of alkaline etching baths on magnesium-rich aluminum can Patented Uec. 22, 1959 be substantially increased by the addition of a small amount of a divalent tin compound.

The rate of attack of an alkaline etching bath on aluminum is dependent to some degree on such factors as the concentration of the alkali and the temperature at which the bath is operated. By'the addition of a divalent tin compound, when etching magnesium-rich aluminum substantially faster etching rates can be obtained than by increasing alkali concentration and temperature alone. However, the additive in no way counteracts the increase in rate obtained by increasing alkali concentration and temperature. Therefore, the divalent tin may, if desired, be used in conjunction with the higher alkali concentrations and temperatures to obtain even faster etching rates.

The accelerating action of a divalent tin compound on the etching rate of an alkaline solution is striking in degree and also in its curious specificity with respect to magnesium-rich aluminum. No accelerating action, or even a slight decrease in etching rate, is observed if other types of alloys are etched in the presence of this additive. However, in the etching of magnesium-rich aluminum the etching rate is almost doubled by the addition of a tin compound.

It is known that the stannc-us ion is soluble in alkaline solution in the form of a stannite. Thus, for example, when stannous chloride is added to a solution of caustic soda, the additive will at first precipitate and then will redissolve as the stannite is formed. The stannite ion is apparently the accelerating agent of the present invention, and therefore the divalent tin may be introduced in any form that will produce the stannite ion in the ai'kaline etching bath. Examples of suitable tin compounds are stannous chloride, stannous sulfate or stannous nitrate.

Most aluminum etching baths are made up using caustic soda, for reasons of economy, although other etching alkalies can be used equally well. imilarly, although caustic soda is the preferred alkali for the practice or" the present invention, other alkalis, such as potassium hydroxide, lithium hydroxide or trisodium phosphate may be used.

In conventional practice, the caustic soda concentration of aluminum etching baths is generally in the range of from 1% to not more than about 20%, most chem milling being done within the range of 8 to 15% since the lower concentrations generally do not contain sufficient caustic to remove the large amount of metal required and at the higher concentrations the baths tend to foam and spatter excessively. Use of the stanno'us ion, however, is not limited to these concentrations, the presence of divalent tin further accelerating the very rapid etching baths of applicants co-pending application Serial No. 544,584, filed November 2, 1955, now abandoned, in which highly concentrated alkaline baths, in the order of 30 to 55% caustic soda are employed.

Several additives are known to reduce or prevent the scale formation that normally accompanies alkaline aluminum etching. Examples of additives suitable for this purpose are those described in US. Patent No. 2,584,017 and applicants co-pending application Serial No. 499,502, filed March 31, 1955, now Patent No. 2,872,301, the preferred scale inhibitors generally being the glucamines, sorbitol and gluconate. Where such scale inhibitors are used they are generally present in amounts of 1 to 5%, based on the amount of alkali. It is preferred to include one or more antiscalant in the fast etching compositions and process of the present invention, although such antiscalant is not necessary for successful acceleration but is included primarily to prevent scale formation. When large amounts of aluminum are to be removed, as is generally the case where very fast etching baths are employed, it is advantageous to prevent the copious scale formation that would ordinarily result from the dissolution of these large amounts. This may be accomplished by addition of sorbitol, for example, in amounts of l to based on the alkali, to a bath containing divalent tin as an accelerant.

It has been found that extremely small quantities of stannous tin have a profound effect on the etching rate of alkaline baths for etching magnesium-rich aluminum. As little as 0.05% by weight of SnCl -2H O based on the alkali increases the etching rate by 5%, and the etching rate increases rapidly on addition of more stannous tin, to a maximum increase of 86% when the tin compound is present in amounts of 2.5% based on the alkali. With even further addition of tin, the rate of etching drops slightly to the still extremely high level of 78% increase over the unaccelerated bath, and remains at this level. It is preferred, therefore, to add sufficient tin to maintain the accelerating action at this high level, a condition which is easily realized by addition of a slight excess above the optimum amount, thereby circumventing the possibility of a drop in etching rate that might occur with continued use and eventual depletion of the bath. In this manner, uniform and very rapid etching rates may be obtained and the uniformity of results retained even after several replenishments of a depleted bath. Calculated on the basis of the weight of the tin ion alone, the range of concentration of tin needed to produce an appreciable accelerating action should be from 0.025 to 3% based on the weight of alkali and preferably should be greater than 1.3%. Amounts greater than 3% tin ion, though having no apparent harmful elfect, do not appear to accelerate the aluminum dissolution further.

In the practice of the present invention, an etching bath for magnesium-rich aluminum containing from 1 to 20% of an etching alkali, is made up with a salt of divalent tin in an amount suflicient to provide at least 0.025% stannous ion based on the alkali. The solid components of the bath may be mixed first or they may be added to the bath separately, in any order. The bath may be operated at temperatures of 120 to 212 F.

In the preferred practice, where a very rapid etch is desired, without going to the high alkali concentrations of copending application Serial No. 544,584, hereinabove identified a relatively strong alkaline etching bath, on the order of 8 to 15% alkali, preferably caustic soda, is employed at relatively high temperatures of about 180 to 200 F. The tin salt is added in amounts suflicient to provide 1.3% of stannous ion or slightly more, based on the weight of caustic. It is also preferred to include from 1 to 5%, based on the caustic, of an antiscalant in the bath, the preferred antiscalants being at least one of the group consisting of gluconate, sorbitol or glucamines. When the aluminum stock has been etched to the desired degree, it is removed, rinsed and then preferably desmutted in a 15 to 20% nitric acid solution.

It is generally preferred to provide all the etching ingredients in a single package in a dry form, the etching solution then being prepared by adding this to water.

In preparing such a composition, etching alkali, preferably caustic soda, is mixed with the accelerant in amounts such that the final composition will contain at least 0.025 part by weight accelerant per 100 parts of caustic. In the preferred practice, sufficient scale inhibitor is included to give a composition containing, in addition to the accelerant about 1 to 5 parts by weight scale inhibitor per 100 parts of caustic.

An example of such a dry composition may contain, for each 100 parts of caustic soda, about 0.05 to 6 or more parts by weight of SnCI -ZH O (corresponding to 0.025 to 3 parts stannous ion) and 1 to 5 parts by weight of sorbitol. In preparing the etching bath the etching composition is added to water to give a solution having the caustic concentration desired, which is generally in the order of 1 to 20%. The stannous chloride dihydrate used in this illustration may, of course, be replaced by other tin compounds such as stannous bromide, stannous sulfate or stannous tartrate.

In preparing the dry formulations just described it is sometimes advantageous to add small amounts of certain other ingredients such as pine oil or nitrates as dedusting or brightening agents, etc.

In order to illustrate more clearly the benefit of the results obtained by the invention, the following example is given:

Example I An etching bath was made up to contain 4% by weight caustic soda and 1% sorbitol, based on the weight of the caustic. When operated at F., this bath etched a panel composed of 52S aluminum at the rate of 0.0126 gram aluminum dissolved per minute per square inch of surface, measured by weight loss. A second bath, precisely similar in'all respects, except that it also contained 2.5% stannous chloride dihydrate based on the caustic, was used to etch a second 528 aluminum panel and gave an etching rate of 0.0234 grams per minute per square inch of surface, an increase of 86%. The two panels were then rinsed, desmutted and compared. The second panel (treated in the accelerated bath) was etched to a depth of 4.5 mils after 10 minutes, and the etched surface was uniform, smooth and shiny. The first panel was etched to a depth of only 2.5 mils after treatment for the same length of time in the unaccelerated bath.

Having thus described my invention, I claim:

1. The method of increasing the rate of etch of a magnesium-rich aluminum alloy by an alkaline etching solution comprising including stannous tin in the solution.

2. The method of claim 1 wherein the tin is present in an amount of at least 0.025% based on the alkali present.

3. The method of etching magnesium rich aluminum alloys comprising treating the surface of said aluminum alloys with an aqueous solution of an etching alkali containing stannous ion at a temperature of at least 120 F.

4. The method of claim 3 wherein said solution also contains an antiscalant.

5. The method of claim 3 wherein said etching alkali is primarily caustic soda.

6. The method of etching magnesium-rich aluminum alloys comprising treating the surface of said aluminum alloys with an aqueous solution containing 1 to 55% of an etching alkali, l to 5% based on the alkali of an antiscalant and at least 0.025% based on the alkali, of stannous tin.

7. The method of etching magnesium-rich aluminum comprising treating the surface of said aluminum with an aqueous solution of caustic soda having a temperature of to 200 F., said solution containing 8 to 15% caustic soda, 1 to 5% based on the caustic soda of at least one antiscalant of the group consisting of sorbitol, glucamine and gluconate, and at least 1.3% based on the caustic soda of stannous tin.

8. The method of etching magnesium-rich aluminum comprising treating the surface of said aluminum with an aqueous solution of caustic soda, said solution containing 30 to 55% caustic soda and at least 0.025% based on the caustic soda of stannous tin.

9. A dry etchant composition for etching magnesiumrich aluminum consisting essentially of dry caustic soda, an antiscalant and a stannous tin compound, said antiscalant being present in amounts of from 1 to 5 parts by weight per 100 parts of caustic soda and said tin compound being present in amounts of from 1.3 to 3 parts by weight per 100 parts of caustic soda calculated on the basis of the weight of tin ion.

10. The composition of claim 9 wherein the antiscalant is one of the group consisting of sorbitol, glucamine and gluconate.

(References on following page) 5 6 References Cited in the file of this patent OTHER REFERENCES UNITED STATES PATENTS Thorpes Dictionary of Applied Chemistry, 4th ed., 1950, vol. X; publ. by Longmans, N.Y.C.; page 855, 2,168,909 Mason Aug. 8, 1939 col. 2, par. 2, under Chemical Properties of Sodium 2,650,875 Dvorkovitz et a1 Sept. 1, 1953 5 Hydroxide, 

1. THE METHOD OF INCREASING THE RATE OF ETCH OF A MAGNESIUM-RICH ALUMINUM ALLOY BY AN ALKALINE ETCHING SOLUTION COMPRISING INCLUDING STANNOUS TIN IN THE SOLUTION. 